1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same, and particularly to an optical scanning device suitable for use in an apparatus such as a laser beam printer or a digital copying machine with an electrophotographic process, which is adapted to reflect and deflect a light beam emitted from light source means with deflection means and optically scans the light beam over a surface to be scanned via a scanning optical system to record image information. More particularly, the present invention relates to a multi-beam scanning apparatus for performing optical scanning using a plurality of light beams simultaneously to thereby realize high speed and high definition, in which satisfactory images with reduced jitter or pitch interval errors can be always obtained, and to an image forming apparatus using the same.
2. Related Background Art
FIGS. 23A and 23B show a perspective view of a conventional multi-beam optical scanning device and a main part schematic view illustrating a light source arrangement of the same, respectively.
A plurality of light beams emitted from light source means 1 are converted into substantially parallel light beams by a condensing lens system 2 to have a light beam width thereof restricted by an aperture stop 3, whereby focal lines longitudinal in a main scanning direction are focused in the vicinity of a deflection facet 5a of deflection means 5 discussed later by a cylindrical lens 4. Reference numeral 5 denotes a polygon mirror serving as deflection means, which is rotated at a constant speed in a direction of arrow A in the figure by drive means 6 for a motor. Reference numeral 7 denotes a scanning optical system having the fθ characteristic, which focuses light beams deflected by the deflection means 5 on a photosensitive drum surface 8 serving as a surface to be scanned and, at the same time, corrects surface toppling of the deflection facet 5a of the deflection means 5.
At this point, two light beams reflected and deflected by the deflection facet 5a of the deflection means 5 are guided onto the photosensitive drum surface 8 via the scanning optical system 7 and optically scan the photosensitive drum surface 8 in a direction of arrow B simultaneously as the polygon mirror 5 is rotated in the direction of arrow A. Accordingly, two scanning lines are formed on the photosensitive drum surface 8 to perform image recording.
In addition, a part of the plurality of light beams deflected by the deflection facet 5a of the deflection means 5 pass through the scanning optical system 7 and are returned by a mirror for synchronous detection 9 to be guided to synchronous detection means 10. Consequently, scanning starting positions of the respective light beams, which are deflected by the deflection means 5, on the surface to be scanned 8 are aligned, whereby satisfactory image recording is performed without misalignment of printing positions in a main scanning direction among scanning lines formed by the plurality of light beams.
As a general optical scanning device, the device employs an optical scanning system called an underfilled system in which a light beam that has a width narrower than that of the deflection facet 5a of the polygon mirror 5 in a main scanning section is made incident upon the deflection facet 5a, thereby optically scanning the light beam over the surface to be scanned 8.
In order to perform highly accurate recording of image information in the multi-beam optical scanning device of this type, it is important that a plurality of light beams focus on the surface to be scanned 8 together and uniformity of jitter (relative misalignment of printing positions of the plurality of light beams in the main scanning direction) and pitch interval (scanning line interval) is satisfactorily corrected over an entire effective area for scanning on the surface to be scanned 8.
In general, when a light beam optically scans the surface to be scanned 8 to form an image, it is necessary to reduce a spot diameter of the light beam on the surface to be scanned 8 and form pitch intervals densely in a sub-scanning direction in order to obtain a high resolution and satisfactory image.
In order to form pitch intervals densely in a sub scanning direction, the light source means 1, in which a semiconductor laser array is arranged to be tilted in an oblique direction with respect to the main scanning direction, is used in many cases.
In this case, since a plurality of light emitting points included in the light source means 1 are arranged in the main scanning direction so as to be spaced at a certain distance apart from each other (FIG. 23B), the respective light beams do not become parallel with each other after exiting the condensing lens system 2 but have a certain angle therebetween. The respective light beams are incident upon the polygon mirror 5 serving as deflection means via the cylindrical lens 4 after exiting the condensing lens system 2. At this point, the respective light beams cross at the position of the aperture stop 3 arranged between the condensing lens system 2 and the polygon mirror 5. An interval among the respective light beams on the deflection facet 5a of the polygon mirror 5 depends upon the angle defined between the respective light beams and a distance from a reference position of the deflection facet 5a of the polygon mirror 5 to the aperture stop 3. It is necessary to constitute the multi-beam optical scanning device such that the respective light beams satisfactorily focus on the surface to be scanned 8 by reducing the interval among the respective light beams on the deflection facet 5a of the polygon mirror 5.
Multi-beam optical scanning devices satisfying such optical characteristics have been conventionally proposed.
JP 5-34613 A discloses a multi-beam optical scanning device in which an aperture stop is arranged between a cylindrical lens and deflection means to reduce an interval among respective light beams in a main scanning direction on a reflection facet of the deflection means. In this example, a plurality of light beams are guided onto a surface to be scanned by a scanning optical system after being made incident on the deflection means via the aperture stop as parallel light beams by a condensing lens system, and optically scan the surface to be scanned simultaneously. In performing the optical scanning, a relationship among the number of light emitting points in a sub-scanning direction of light source means, a pitch of the light emitting points, a distance from the deflection means to the aperture stop, and a focal length of the condensing lens system are specified to cause the plurality of light beams to satisfactorily focus on the surface to be scanned and reduce curvature of field.
In addition, JP 2001-228422 A discloses a multi-beam optical scanning device in which a relationship among an interval among the light emitting points of light source means, a focal length of a collimator lens, a distance from an aperture stop to a deflection facet of deflection means, a focal length of a scanning optical system, and the number of pixels per one inch in a main scanning direction on a surface to be scanned is appropriately set, whereby jitter is reduced.
However, since there is a limitation on the arrangement of the aperture stop, there is a problem in that a degree of freedom of an arrangement of optical elements is narrowed.
JP 2000-292721 A discloses an example of an overfilled optical system which uses a light source having two light emitting points arranged so as to be spaced apart from each other in a main scanning direction. In this invention, a light beam magnification optical system is provided between light source means and deflection means, thereby securing a necessary amount of light of deflected light beams which scan a surface to be scanned. Therefore, there is neither a description concerning jitter and a pitch interval error nor a description concerning synchronous detection means provided herein. Thus, this invention does not satisfy the structural requirements for reducing jitter.
In addition, JP 11-249040 A discloses an example of an overfilled optical system in which two light emitting points are arranged at an interval of 14 μm in a sub-scanning direction. With the structure of this invention, a lateral magnification in a sub-scanning direction of all optical systems as a whole, which are provided between light source means and a surface to be scanned, is determined uniquely in accordance with an interval among scanning lines. Thus, there is no degree of freedom of design. In addition, since it is necessary to set a sub-scanning magnification of a scanning optical system to a relatively small value and thus an imaging element having a power mainly in the sub-scanning direction is arranged near a surface to be scanned, there are a problems in that the multi-beam optical scanning device is enlarged in size and costs required for manufacturing the multi-beam optical scanning device are increased.
It is necessary to solve the above-mentioned problems and satisfactorily correct jitter and a pitch interval error. Jitter means relative misalignment of printing positions of a plurality of light beams in a main scanning direction. A pitch interval error means deviation of an interval among scanning lines, which are formed when a plurality of light beams scan a surface to be scanned simultaneously, from a specified values (e.g., if a pixel density is 600 dpi, a scanning line pitch is 42.3 μm).
Jitter which is characteristic of a multi-beam optical scanning device includes drum oblique-incidence jitter, wavelength difference jitter, defocus jitter, and the like, which have different causes. The cause of the drum oblique-incidence jitter is a difference in an optical path length for each of a plurality of light beams, which occurs because the plurality of light beams are made incident on a cylindrical drum surface while forming certain angles with respect to a sub scanning direction. This jitter increases toward a peripheral part of an effective area for scanning from an optical axis of a scanning optical system. The cause of the wavelength difference jitter is a chromatic aberration of magnification due to occurrence of a wavelength difference among a plurality of light beams. With this jitter, misalignment of printing positions due to the chromatic aberration of magnification within an effective area for scanning and misalignment of scanning starting positions due to the chromatic aberration of magnification on synchronous detection means occur simultaneously. The defocus jitter is jitter due to a difference in focus positions in a main scanning direction on a surface to be scanned and on synchronous detection means.
The cause of this defocus jitter is that a plurality of light beams reach positions spaced apart from each other in a main scanning direction on the scanning optical system when the plurality of light beams scan an identical position on the surface to be scanned. Thus, in a multi-beam optical scanning device in which a plurality of light emitting points of light source means are arranged in parallel with each other in a sub-scanning direction without being spaced apart from each other in a main scanning direction as in FIG. 21B, since a plurality of light beams pass through an identical optical path in the main scanning direction, the defocus jitter does not occur.
However, since an interval among light emitting points is fixed in a monolithic multi-beam light source means, in the case of the multi-beam optical scanning device in which a plurality of light emitting points are arranged in parallel with each other in the sub-scanning direction, a lateral magnification in the sub-scanning direction of all the optical systems provided between the light source means and the surface to be scanned is determined uniquely in order to set an interval among scanning lines to a specified value. There is a problem in that this makes a degree of freedom of design extremely low. Usually, an interval among light emitting points is 90 μm or 14 μm. In the case in which the plurality of light emitting points are arranged to be parallel with each other in the sub-scanning direction, it is necessary to set the lateral magnification in the sub scanning direction of all the optical systems to a relatively small value of 0.47 or 3.02 in order to set the interval among scanning lines to 42.3 μm which is equivalent to 600 dpi. In order to set the sub-scanning magnification of all the optical systems to a small value, a long optical element having a power in the sub-scanning direction is required to be provided in the scanning optical system in the vicinity of the surface to be scanned. There is a problem in that this makes enlargement of the multi-beam optical scanning device inevitable.
In addition, since the long optical element is expensive, costs for manufacturing the multi-beam optical scanning device are increased.